Tests/RunCMake/export: handle collapsed newlines

When the build path is long with a space near the line length, a newline
is lost.

Tests/RunCMake/export/DependOnDoubleExport-stderr.txt

@@ -3,8 +3,7 @@ CMake Error in CMakeLists.txt:
   that is not in this export set, but in multiple other export sets:
   .*/Tests/RunCMake/export/DependOnDoubleExport-build/exportset.cmake,
   .*/Tests/RunCMake/export/DependOnDoubleExport-build/manual.cmake.
-
-
++
   An exported target cannot depend upon another target which is exported
   multiple times.  Consider consolidating the exports of the "doubleexported"
   target to a single export.